1. Field of the Invention
The present invention relates to a perovskite manganese oxide thin film and a manufacturing method therefor. More specifically, the present invention relates to a perovskite manganese oxide thin film the electrical, magnetic or optical properties of which are switched in response to a stimulus such as temperature, electrical field, magnetic field or light exposure, and to a manufacturing method therefor.
2. Background of the Related Art
There has been concern in recent years that semiconductor devices may be facing the limits of the scaling law, which has been a guiding principle of performance advances in the field. In this context, materials are being developed that will make new operating principles possible in order to weather the crisis when the transistor limit is reached. For example, in the field of spintronics, which exploits the spin degrees of freedom of electrons, there has been development aimed at high-density non-volatile memories capable of high-speed operation at the same level as DRAM (dynamic random access memory).
There has also been progress in research into materials having strongly-correlated electron systems that cannot be described in terms of band theory, which is the cornerstone of semiconductor device design. Substances have been discovered that exhibit very large and rapid changes in physical properties caused by phase changes in the electron system. In strongly correlated electron system materials, a variety of electron phases with a variety of orders formed by spins, charges and orbitals are possible because the phase state of the electron system is affected not only by the spin degrees of the freedom but also by the degrees of freedom of the electron orbitals. Typical examples of strongly correlated electron system materials are the perovskite manganese oxides, in which a first order phase transition produces a charge-ordered phase by alignment of 3d electrons of manganese (Mn) and an orbital-ordered phase by alignment of the electron orbitals.
In a charge-ordered phase or orbital-ordered phase, electrical resistance increases because the carrier is localized, and the electron phase becomes an insulator phase. The magnetic behavior of this electron phase is that of an antiferromagnetic phase due to the double exchange interactions. The electron states of the charge-ordered phase and orbital-ordered phase should often be regarded as semiconductor states. This is because although the carrier is localized in the charge-ordered phase and orbital-ordered phase, the electrical resistance is lower than that of a so-called band insulator. In accordance with convention, however, the electron phases of the charge-ordered phase and orbital-ordered phase are here called insulator phases. Conversely, when the behavior is metallic with low resistance, the electron phase is a ferromagnetic phase because the spins are aligned. The term “metallic phase” is defined in various ways, but here a metallic phase is one in which “the temperature derivative of resistivity is positively signed”. Expressed in this way, the aforementioned insulator phase can be re-defined as one in which “the temperature derivative of resistivity is negative”.
A variety of switching phenomena have reportedly been observed in bulk single-crystal materials made of substances capable of assuming either the aforementioned charge-ordered phase or orbital-ordered phase, or a phase that combines both a charge-ordered phase and an orbital-ordered phase (charge- and orbital-ordered phase), see Patent Documents 1 to 3, i.e., Japanese Patent Application Publication Nos. H08-133894, H10-255481, and H10-261291. These switching phenomena occur in response to applied stimuli, namely, temperature changes around the transition point, application of a magnetic or electric field, or light exposure. These switching phenomena are typically observed as very large changes in electrical resistance and antiferromagnetic-ferromagnetic phase transitions. For example, resistance changes by orders of magnitude in response to application of a magnetic field are a well-known phenomenon called colossal magnetoresistance.
To achieve any device with a high degree of utility using a perovskite manganese oxide, the switching phenomena must be manifested at room temperature or above, such as an absolute temperature of 300 K or more. However, the switching phenomena disclosed in the aforementioned documents of prior art have all been verified only under low-temperature conditions of about liquid nitrogen temperature (77 K) or less for example. In the perovskite manganese oxides disclosed in the aforementioned documents of prior art, trivalent rare earth cations (hereunder represented as “Ln”) and a divalent alkaline-earth (“Ae”) randomly occupy the A sites in the crystal structure of the perovskite. It is thought that the temperature at which the switching phenomena are manifested is lowered as a result of this randomness. In fact, it is known that the transition temperature for the charge-ordered phase can be elevated to about 500 K by ordering the A-site ions in an AeO-BO2—LnO-BO2—AeO-BO2—LnO-BO2 . . . configuration. Regular arrangement of the ions occupying the A sites as in this example is called “A-site ordering” below. A feature of the group of substances exhibiting such high transition temperatures is that they contain Ba (barium) as an alkaline-earth Ae. For example, transition temperatures above room temperature have been reported with substances containing Ba as an alkaline-earth Ae and using Y (yttrium), Ho (holmium), Dy (dysprosium), Tb (terbium), Gd (gadolinium), Eu (europium) and Sm (samarium) as rare earths Ln with small ionic radii.
For these phenomena to be used in an electronic device, magnetic device or optical device, the switching phenomena must also be manifested when the substance is in the form of a thin film, Conventionally, the problem has been that if a single-crystal thin film of a perovskite manganese oxide is prepared by deposition on a (100) oriented substrate, the switching phenomena are not manifested in the resulting (100) oriented perovskite manganese oxide single-crystal thin film. This is because the in-plane crystal lattice of the single-crystal thin film is fixed to the in-plane crystal lattice of the substrate, and the first orderphase transition to a charge-ordered phase or orbital-oriented phase requires a kind of lattice deformation called Jahn-Teller deformation, which is suppressed by in-plane fourfold symmetry of the substrate. On the other hand, Patent Document 4, i.e., Japanese Patent Application Publication No. 2005-213078, discloses a perovskite oxide thin film formed using a substrate with a (110) orientation. According to this disclosure, the formed thin film allows shear deformation of the crystal lattice during switching when the in-plane fourfold symmetry of the (110) oriented substrate is broken. That is, in a thin film formed in accordance with Patent Document 4 the crystal lattice is oriented parallel to the substrate plane, while the charge-ordered plane or orbital-ordered plane is non-parallel to the substrate plane. As a result, first order phase transitions accompanied by deformation of the crystal lattice are possible even with a single crystal thin film in which the in-plane crystal lattice is fixed to the in-plane lattice of the substrate. Thus, according to Patent Document 4, a transition or in other words a switching phenomenon at high temperatures equivalent to those obtained with the bulk single crystal can be achieved by using a (110) oriented substrate.
Furthermore, Patent Document 5, i.e., Japanese Patent Application Publication No. 2008-15618, discloses an example of a thin film formed from a perovskite manganese oxide with A-site ordering as discussed above. According to Patent Document 5, a thin film in an amorphous state was first formed by a photo-assisted deposition process, and then laser annealed to achieve crystallization and A-site ordering. Specifically, A-site ordering of a SmBaMn2O6 thin film formed on a (100) oriented SrTiO3 (lattice constant 0.3905 nm) substrate was confirmed by electron beam diffraction.
However, switching is suppressed in a single-crystal thin film of perovskite manganese oxide formed on a (100) substrate. As a result, even if a substance or material exhibiting a charge-ordered phase within a temperature range suited to practical use (such as room temperature) can be prepared using a bulk single crystal, it cannot immediately be applied to a device. Patent Document 5 does not disclose whether or not the thin film subjected to A-site ordering is a single-crystal thin film, but supposing it to be a polycrystalline film, or in other words a film comprising multiple grains with different crystal orientations on the same substrate, A-site ordering and charge and orbital ordering would then be impeded by lattice defects in the thin film. Thus, in the substance formed as a thin film in Patent Document 5 there is a concern of a decreased transition temperature or even the loss of the first order phase transition itself in extreme cases.
As in the case of ordinary semiconductor devices, a defect-free single-crystal thin film is necessary in order to achieve high-performance switching properties without variation in a single-crystal thin film of a perovskite manganese oxide. One possible way of doing this is by using a (110) oriented substrate as disclosed in Patent Document 4 and the like. In a (110) oriented thin film, the atomic plane is (Ln,Ba)BO—O2—(Ln,Ba)BO. This describes a layered atomic structure obtained by forming one atomic layer consisting of A sites containing Ba atoms or a rare earth element Ln, B sites, and O atoms, and then forming an atomic layer containing two 0 atoms. Thus. A-site ordering must be achieved within a plane parallel to the atomic plane. However, some factor must provide a driving force for ordering the A-sites within the plane. In fact no such factor exists, and ordering the A-sites of a (110) oriented thin film is not an easy matter.
As discussed above, the problem has been that a charge-ordered phase capable of manifesting switching phenomena at a temperature range of room temperature and above cannot be achieved in a thin film of perovskite oxide simply by combining prior art. The present invention, which was developed in light of the problems described above, contributes to the preparation of various devices using perovskite manganese oxide thin films by (1) allowing first order phase transitions, while at the same time (2) achieving A-site ordering, both features which are necessary to achieve switching that operates at room temperature.
The inventor of this application examined these problems closely, focusing on the relationship between the atomic plane of the thin film and the symmetry within that plane. The inventor then discovered means for solving these problems.